This year my son asked to be a Lego Rock Monster for Halloween.  I thought,  sure I can do that.  WOW it took way longer than i thought it would, but here are the instructions so maybe it won't take you as long.
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Step 1: Materials

Picture of Materials
Here is what I used to make this costume

Blue roll of semi clear wrapping
Styrofoam sheet from HDepot
Grey sweat shirt and pants
Hot glue - lots of it
Grey spray paint
Duct tape

Hot glue gun
Sand paper
Misc. shop tools

Step 2: The build

Begin by getting your hands on the Lego rock monster toy.  I'm guessing if your kid wants to be this you already have a few.  Then cut out and form a cardboard tub to fit your child.  Determine the amount of space needed for the head opening and then cut a Styrofoam ring for the top to hold the cylinder shape.

Draw on and cut out the hands from the foam. Stack 3 sheets high and cut out area for pringles tube.  Hot glue pieces together and repeat for other hand.
Cut to size and hot glue a cardboard insert pieced for the hands.  This adds strength and helps look more like the toys hand.  Roughly shape the hands to look like the toy.

Cut out a few circles and glue together to start building up the head piece.
Take measurements of your child's head and build up as needed.
I shaped out the inside of head piece and lined it with cardboard strips (keeps Styrofoam out of the hair and looks cleaner)
Rough cut rock looking shapes using a Dremel or other tool.  This is a good place to use the toy for a reference.  
I recommend reinforcing the inside mouth opening with cardboard strips and then cut out teeth shapes.

Stack and glue up 2,3 and 4 high curved pieces of different shapes for rocks.  Sand at different angles like the toy.  Where a mask and glasses because this foam can't be good for you to breath and it gets everywhere.